Recently, attention has been paid to various methods of utilizing fine bubbles having a diameter of micrometer level or nanometer level, and various apparatus for generating fine bubbles have been proposed. The present invention relates to an apparatus having a cylindrical interior space, wherein a gas-containing liquid is introduced into the interior space to generate a swirling flow to break up the gas into fine bubbles by the shear force of the swirling flow.
An apparatus of the type described above is disclosed, for example, in Japanese Patent Application Publication No. 2001-276589. The apparatus disclosed in this publication has a cylindrical swirling flow generating member immersed vertically in a liquid in a liquid storage tank for generating fine bubbles. The swirling flow generating member has one inlet hole provided in an upper end portion thereof to introduce a gas-liquid mixed fluid. The inlet hole tangentially intersects the inner peripheral surface of the cylindrical member. A funnel-shaped portion is provided at the lower end of the swirling flow generating member. The funnel-shaped portion has a fluid outlet at the lower end thereof. A pipe is connected to the inlet hole to supply a pressurized fluid from a pump. An aspirator is connected to an intermediate portion of the pipe. A liquid mixed with air as bubbles is passed through the aspirator and introduced into the swirling flow generating member through the inlet hole. The introduced gas-liquid mixed fluid moves downward while swirling in the swirling flow generating member and is discharged into the liquid storage tank through the fluid outlet at the lower end of the swirling flow generating member. A shear force occurs between the discharged gas-liquid mixed fluid and the liquid in the storage tank. Thus, the bubbles in the gas-liquid mixed fluid are broken up into fine bubbles.
Japanese Patent Application Publication No. 2003-117368 discloses a fine bubble generating apparatus including a cylindrical member having an inner peripheral surface formed into a cylindrical shape or the like. In this apparatus, a liquid, together with air, is introduced into a pump for liquid supply to form a gas-liquid mixed fluid containing bubbles in the pump, and the gas-liquid mixed fluid is introduced into the cylindrical member through one inlet hole provided in the cylindrical member close to one end thereof. The introduced gas-liquid mixed fluid moves axially toward the other end of the cylindrical member while swirling and is discharged through an outlet provided at the other end of the cylindrical member. Thus, the bubbles in the gas-liquid mixed fluid are broken up into fine bubbles. This publication also discloses an apparatus wherein a gas-liquid mixed fluid is introduced into a cylindrical member through an inlet hole provided in an axially central portion thereof, and the introduced gas-liquid mixed fluid moves toward both ends of the cylindrical member while swirling and is discharged through outlets provided at both ends of the cylindrical member.
Japanese Patent No. 3682286 discloses a bubble breaking-up apparatus including a swirling flow generating member with an oval or elliptical inner peripheral surface that is immersed in a liquid in a liquid storage tank. In this apparatus, the swirling flow generating member has one inlet hole for introducing a gas-liquid mixed fluid at the center of the major axis of ellipse. The gas-liquid mixed fluid introduced through the inlet hole moves in a swirling flow toward outlets provided at both ends in the major axis direction of the swirling flow generating member and is discharged through the outlets. It is stated that the gas in the gas-liquid mixed fluid is broken up into fine bubbles by swirling flow in the swirling flow generating member and shear force applied to the gas-liquid mixed fluid when discharged.
Japanese Patent Application Publication Nos. 2002-11335 and 2002-166151 disclose an apparatus wherein a gas-liquid mixed fluid is introduced into a swirling flow generating member having a cylindrical inner peripheral surface through two axially spaced gas-liquid mixture inlet holes provided in the swirling flow generating member, and the introduced gas-liquid mixed fluid is discharged through outlets provided at both ends of the swirling flow generating member.    Patent Document 1: Japanese Patent Application Publication No. 2001-276589    Patent Document 2: Japanese Patent Application Publication No. 2003-117368    Patent Document 3: Japanese Patent No. 3682286    Patent Document 4: Japanese Patent Application Publication No. 2002-11335    Patent Document 5: Japanese Patent Application Publication No. 2002-166151